1. Field of the Invention
This invention relates to thermocouples and in particular to bimetallic thermocouple sensors composed of metals and alloys having improved reliability and performance characteristics in high-temperature, high-vacuum systems. Thermocouple sensors of the present invention are formed from molybdenum, iridium and alloys thereof to provide a reliable temperature sensing device in high-temperature and vacuum systems. The chemical and physical compatibility of molybdenum with iridium and alloys containing only those two metals provide thermoelectric sensors having reduced thermocouple hot junction failure upon initial heating and emf-temperature performance changes resulting from preferential evaporation of one of the thermocouple elements in vacuums and extended periods of use at temperatures between 1500.degree. and 2400.degree. C.
The thermocouple sensors of the present invention are particularly well suited to bare metal sensor applications in high-temperature, high-vacuum systems but can also be sheathed and insulated for applications in systems up to about 2450.degree. C. Whether sheathed or unsheathed the thermocouples of the present invention provide stability and reliability by utilizing molybdenum and iridium and alloys containing only those two metals in the bimetallic thermocouple sensor to provide a reliable thermocouple having an improved operational life and reliability.
2. Description of the Prior Art
Thermocouples are useful temperature measuring devices which measure temperature by employing dissimilar metal conductors joined at a point where the temperature is to be measured with the free ends connected to an instrument which measures the amount of voltage generated at the junction of the dissimilar metal conductors. The thermocouple sensor or the bimetallic junction of the dissimilar conductors have been formed of various metals such as copper and iron which provide a thermoelectric differential between the two metals upon exposure to heat. These and other metals employed in prior art thermocouple sensors have had the disadvantage of melting at fairly low temperature, i.e., copper, 1083.degree. C., and have required insulation and various sheathing systems to protect the thermocouple during operation at prolonged elevated temperatures and sometimes have resulted in undesirable reactions between the metals in the sensor.
The problems of undesirable reactions in thermocouple sensors have been aggravated by the temperatures encountered in nuclear reactor systems, rocketry heat sensors, high-temperature and vacuum processing and other applications where temperature measurements at or above 1500.degree. are involved. Thermocouples employing bimetallic sensors of tungsten and rhenium with sheathing and insulation have been utilized in an effort to prevent the disintegration of the thermocouple in such systems. The insulation and sheathing systems have had the disadvantage of resulting in time delays in obtaining temperature readings due to the insulation and mechanical failure resulting from such problems as gas leakage at the thermocouple sheath seals, cracked sheaths and other mechanical limitations imposed by ceramic insulated metal sheathed thermocouple sensors.
The tungsten, rhenium thermocouple-sensor combination and other prior art bimetallic bare sensor combinations have generally not proven to be uniformly reliable or to have a useful operational life at temperatures above 1800.degree. C. due to breakage of the thermocouple hot junction upon initial heating and drifts in emf, temperature relationships. These problems are believed to be the result of thermal and chemical phase transitions and of preferential evaporation of one of the metals in the bimetallic sensor. In particular the tungsten, rhenium thermocouple combination has not proven reliable in high-temperature and vacuum systems as a result of high-vapor-pressure differential between rhenium and tungsten. This high-vapor-pressure differential causes thermocouples comprised of these substances to drift in their emf, temperature relationship as a result of preferential losses of rhenium by evaporation at elevated temperatures, particularly in high-vacuum systems.
The problems associated with thermocouples employing bitmetallic sensors of tungsten and rhenium are greatly reduced in thermocouples employing bimetallic sensors of tantalum and rhenium which are the subject of my copending patent application Ser. No. 629,457 filed Nov. 6, 1975 and issued as U.S. Pat. No. 4,045,247. The vapor pressures of tantalum and rhenium match closely, and the thermal limits for such thermocouples approach the melting points of these two metals which range from about 2690.degree. C. to about 3000.degree. C. for high tantalum solid solutions and from about 2755.degree. C. to about 3180.degree. C. for high rhenium solutions.
A desirable matching of vapor pressures also holds for the constituents of thermocouples employing bimetallic sensors employing platinum and rhodium. However, the 1770.degree. C. melting point of platinum restricts the utility of these thermocouples to about 1700.degree. C. or lower.